This invention relates to a new carbon based material, its manufacture and use. More particularly, the invention relates to a carbon based material produced from the consolidation of amorphous carbon under elevated temperature compression having a broad range of applications, such as for example, as electrode material and as structural material.
Carbon is a solid element that exists in many forms. Solid carbon can have a tetrahedral crystalline array (diamond) or hexagonal graphine planes. If the graphite planes are arranged in planar formations, the resulting solid is known as graphite. If the graphine planes are more randomly arranged, the resulting form of carbon is known as amorphous carbon. Activated carbon, carbon black and charcoal are examples of amorphous carbon. With respect to crystallinity, graphite has short range and long range order, while amorphous carbon has only short range order in the graphine planes. This difference is manifested in their surface properties with amorphous carbon being more reactive than graphite. The difference is also manifested in the spectral patterns generated when the material is tested by x-ray diffractionxe2x80x94graphite spectra show ordered crystal patterns, while the amorphous material pattern has no discernible pattern.
One form of amorphous carbon, activated carbon, is manufactured from an organic source material. Typically, activated carbon is made through carbonization of organic materials, such as wood, coal, pitch, coconut shells, petroleum, animal bones, etc., followed by an activation process. During the activation process, some of the surface platelets are burned out leaving behind many pores with different shapes and sizes, hence activated carbon with an increased surface area and porosity is generated. In general, the pore size plays a role in determining the properties of the activated carbon for various applications. According to IUPAC definitions, pores can be characterized as macropores with pore diameters above 50 nm, mesopores with pore diameters between 2-50 nm, and micropores with pore diameters below 2 nm. In addition to its porosity, activated carbon is conductive and usually inert in many aqueous and organic systems.
Because of its porosity, activated carbon has been widely used in various industries as an adsorbent. The most commonly seen applications include deodorizing, decoloring of gas or liquid phase substances, and removing of toxic organics/inorganics from air and water. The mining industry uses activated carbon for the recovery of precious metals like gold from leaching solutions. Typically, activated carbon is packed into a column through which the gas or liquid to be treated is percolated continuously. The adsorption process takes place at the interface between the carbon phase and the fluid phase.
Its large specific surface area, porosity, conductivity and inert nature make it suited for use as an electrode in electrochemical applications such as energy storage devices and water deionization/desalination devices. The underlying principles of these electrochemical electrodes are rooted in the way that dissolved ions in water behave next to charged solids. Salt dissolves in water forming an electrolyte solution which has no net charge, that is, the net cationic charge will exactly equal the net anionic charge. When a charged solid (i.e., a particle, plate, etc.) is placed in such a solution, the ions of the electrolyte distribute in a manner that will minimize the charge density through a layer known as the electric double layer. Counter ions will be more concentrated within layers nearest the charged surface, but the concentration will gradually decay to equal ion charge in the bulk. A capacitor is formed between the charged surface and the net zero potential of the bulk. A typical value for this capacitance is on the order of 10 xcexcF/cm2 of surface area.
If two electrodes arc placed in an electrolyte solution with an applied potential, the ions will partition so that the cations will migrate to the cathode to fill one double layer, and the anions will migrate to the anode and fill the other double layer. The separation of the cationic and anionic species in this manner is a means to store energy (ultracapacitors) or a means to desalinate water (capacitive deionization). Ultracapacitors have been studied as a potential storage mechanism in applications that require large energy storage devices capable of rapid energy discharge. The primary interest of these devices has been in electric automobiles and electronic devices. Capacitive deionization technology is recently being used in treating brackish water and seawater.
The basic operating principles of carbon electrodes are readily understood, but the manufacturing techniques for producing activated carbon electrode material have been limited. Three processes are currently used, identified by the types of materials they employ as feedstock: granular activated carbon, carbonization of polymers, and carbon aerogels.
Early in the 1950""s, researchers started to use granular activated carbon to make electrodes for electrochemical studies. Because carbon particles cannot consolidate under normal conditions, it is thought necessary to either apply high pressure or some kind of binder to keep the carbon particles in contact in order to form an electrode. It is difficult to make such an electrode that is maintained under constant high pressure, the system would be unacceptably bulky and dangerous. Thus, most studies have been carried out on carbon electrodes with an organic or polymeric binder mixed together with the carbon powders. The binders can be organic polymers, clays, or inorganic chemicals. Disadvantages exist with the use of binders to form the electrodes. Binders block a large portion of carbon surfaces, causing some pores to be blinded, and occlusion therefore is inevitable, thus lowering the available surface area of the carbon. Binders also deteriorate the conductivity of the electrodes because most binders are themselves nonconductive. The contamination from the binders also hinders their uses in electroanalytical applications.
Modern carbon electrodes are manufactured from phenolic resins or other types of resins by a process in which the resin is preformed to a certain shape then subjected to high temperatures for extended periods of time until complete carbonization occurs. The resulting carbon has relatively large surface area, but the manufacturing technique requires the use of toxic and environmentally dangerous chemicals. Often, organic solvents and aromatic compounds, such as benzene and toluene, are evolved during the manufacturing process. The volume of carbon formed is considerably smaller than the original resin size which leads to low product yield. This is a significant problem if specific geometric shapes or sizes are required. This manufacturing technique also has the disadvantages of high material cost and weak material strength due to the xe2x80x9cshrinkingxe2x80x9d of the precursor carbon at high carbonization temperatures.
Some specific carbon electrodes are manufactured from aerogel compounds with sol-gel technology by similarly carbonizing organic compounds. Resorcinol-formaldehyde, for instance, can be infiltrated into a conductive substrate or formed into a solid. Solvents may be rinsed through the material prior to pyrolization in an inert atmosphere, such as in argon or nitrogen. The pyrolysis process produces a, vitreous carbon material which has a high surface area and high electrical conductivity. However, this manufacturing technique includes extremely high manufacturing costs and leads to the release of organic solvents such as acetone, formaldehyde and aromatic compounds as the substrate is thermally changed to carbon. These can pose serious health hazards to workers near the furnaces. The final shape of the carbon materials is much smaller than the feed material. Additional processing would be required to produce a specific geometric shape.
Thus, there exists a need for a more efficient, less expensive, more environmentally friendly process to manufacture activated carbon electrodes.
With respect to ultracapacitors, in the early 1980s, technology was developed to make an ultracapacitor of very large capacitance, on the order of Farads. Normal capacitors have a pico- to micro Farad capacity. As high-energy storage devices, ultracapacitors can be used as load leveling devices for electric and hybrid vehicles, memory backup for computers, as well as applications in areas such as portable communications, pulse energy systems and actuators. With the development of electrical and electronic technology, demands for high-performance energy storage devices have emerged and have kept growing.
The idea of ultracapacitors is based on the theory of the electrical double layer. An electrical double layer is the ionic layer developed at the interface between a charged solid and an electrolyte. When a potential is applied over two electrodes in an electrolyte solution, electrical double layers are developed and a charge separation is obtained by building up of ions of opposite signs with the electrode. If electrodes are polarizable, a final charge state will be reached at equilibrium. Since an electrical double layer is essentially a charge separation layer, it behaves as an electrical capacitor. Accordingly to the double layer theory, the capacitance of an electrical double layer depends on the charges stored in the double layer and the permitivity of the solvent within the double layer region. Typically, the specific capacitance of a double layer is on the order of 10 xcexcF/cm2. Much effort has been made to make ultracapacitors with various forms of activated carbon. Although prototype and commercial ultracapacitors have been made with activated carbon, overall performance has not been satisfactory mainly due to the inevitable problem of occlusion from binders used or the high cost of material manufacturing.
With respect to capacitive deionization, by taking advantage of the very high surface area of activated carbon, ions can be xe2x80x9cstoredxe2x80x9d in electrical double layers when a potential is applied across two activated carbon electrodes, even though these ion species have no affinity to activated carbon in the absence of the applied potential. Once the electrodes are grounded or the polarity is reversed, the double layers are relaxed/reversed, then the stored ions are released back to the bulk solution. Therefore, a coupled deionization and regeneration process can be achieved. Previously, either an inert polymeric binder was used to form a block electrode or a membrane was used to constrain the carbon particles. As a consequence, the electrical and mass transfer resistance is very high and the overall performance is poor. It is clear that a block type electrode without a binder is greatly desired if activated carbon is going to be used for such electrochemical applications. It is obvious that a highly conductive monolithic activated carbon material with high surface area, larger macropore size and of lower cost is greatly desired for effective desalination/deionization.
Turning now to the use of amorphous carbon in producing structural materials, in the materials industry, few forms of carbon are useful for fabricating parts. Graphite is most commonly used in applications requiring conductive materials with high strength and low density, such as in various high temperature casting molds or electrode materials. Graphite can also be an admixture to improve the properties of other materials. Carbon reinforced with graphite fibers is a relatively new material that has found broad uses in lightweight structural material, sporting equipment, such as bicycle frames, golf clubs and tennis racquets, and by NASA for use in space vehicles such as the shuttle. These materials have unique high temperature strength properties which retain stiffness and strength even at temperatures exceeding 1650xc2x0 C. These are very expensive materials because of the complex manufacturing process. Carbon fibers are mixed within resins, then pyrolyzed to generate the carbon-matrix materials around the carbon fiber reinforcement. These materials are then subjected to a long and complicated densification process known as chemical vapor deposition to produce the final product.
Therefore, there exists a need for a more efficient, less expensive process to manufacture carbon structural materials.
In the 1950""s, a metallurgical process called hot isostatic pressing (HIPing) was introduced into the area of metallurgy. HIPing involves the isostatic application of a high pressure gas at an elevated temperature in a specifically constructed vessel. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and weld up in a process known as sintering. Encapsulated powder and sintered components alike are densified and consolidated. It is typical to operate a HIP at temperatures of 1000-3000xc2x0 C. and pressures of 25,000-60,000 psi. Cold isostatic presses (CIPs) have also been developed which typically apply an isostatic pressure to a material at or near room temperature.
The present invention is a novel carbon based material and process for its production which takes advantage of the properties of amorphous carbon to produce a vastly improved material which has broad applications. The process incorporates consolidation of amorphous carbon under elevated temperature compression. The products of the process have unique chemical, electrical and physical characteristics.
The novel carbon based material of the present invention is versatile so as to be used in a broad range of applications such as in the manufacture of structural materials and of electrode materials. The process of the present invention is an inexpensive manufacturing method that produces materials that are near net shape or are readily machinable to specifications and the process is effective at generating monolithic carbon material without the use of binders, or any noxious or toxic chemicals. Carbon source material can be selected based on any combination of properties such as available surface area, particle size distribution, and conductivity to produce material with optimal properties for the specific application desired. Additionally, the process parameters can be optimized to produce specific material properties, such as degree of densification, internal porosity, available surface area, or other property that the end user may require. The process of the present invention provides for the making of large billets of activated carbon so that production costs could be reduced.
After consolidation at elevated pressures and temperatures, novel carbon material can be produced with desired surface areas, porosity, density, strength and resistivity. Cyclic voltammetry (CV) curves demonstrate that the novel material is stable over a wide potential range in aqueous solution and therefore suitable for electrochemical applications. A capacitive feature of the CV curves indicates that the novel material is capable of storing a great amount of charge. The novel material is suitable for application of ultracapacitors. For example, test cells using electrodes of the novel material demonstrated that the capacitor had a specific capacitance of 53 F/g in an aqueous electrolyte and 23 F/g in an organic electrolyte, based upon the electrode material only. Electrodes of the novel material can be used for deionization, such as desalination. Such electrodes are effective at removing ions at a low energy consumption rate.
It is a feature of the present invention to provide a novel material made of amorphous carbon consolidated under elevated temperature and pressure.
It is another feature of the present invention to provide a manufacturing process for the production of said novel carbon based material.
It is another feature of the present invention to provide a manufacturing process whose parameters can be altered to obtain the novel material having optimized characteristics for a particular application.
It is another feature of the present invention to provide a said novel material for a road range of applications.
It is another feature of the present invention to provide an electrode made from the novel material.
It is another feature of the present invention to provide an activated carbon electrode made from the novel material.
It is another feature of the present invention to provide for the application of this novel material for use in the desalination of water.
It is another feature of the present invention to provide for the application of this novel material for use in ultracapacitors.
It is another feature of the present invention to provide the application of this novel material for use in the removal of solids from water in a manner of dewatering slurries or separating different solids in suspensions.
It is another feature of the present invention to provide the application of this novel material for use in the direct electroplating of metal from aqueous and non-aqueous electrolyte solutions.
It is another feature of the present invention to provide the application of this novel material in the deionization of water.
It is another feature of the present invention to provide the application of this novel material in environmental processing for the direct electrochemical destruction of pollutants and contaminants such as from water.
It is another feature of the present invention to provide the application of this novel material in water treatment, such as water softening and pH control.
It is another feature of the present invention to provide said novel material for use as a carbon structural material in a broad range of applications.
It is another feature of the present invention to provide the parameters of the manufacturing process for production of the novel material that could be used as a carbon-based composite or carbonaceous structural material.
It is another feature of the present invention to provide the application of this novel material for uses in highly corrosive or chemically active environments.
It is another feature of the present invention to provide the application of this novel material for uses in high temperature applications.
It is another feature of the present invention to provide the application of this novel material in uses in applications requiring materials of high strength, low density and/or specific porosity.
Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following drawings, detailed description and claims.